I think ~40C is ok for stock clocks and voltages and ~50-55C if
overclocked.
My newcastle 2.4ghz is running 38C idle (room temp 75F).
Max gets up to 59-60c with old copper thermalright heatsink and
2900rpm 92mm fan dialed down to 2500rpm for noise reduction.
those .13 um cores run hot.
FYI, the 2.2ghz 754 venice core oem with the SSE3 extensions runs
very cool and is faster than the 2.4ghz cpu at things that use those
SSE3 extensions. It may be cheaper than some heatsinks at $49 or
so.